Fluorescent contrast agents have been developed and/or employed that enhance the optical detection of diseased tissues. These agents excite and re-emit at near-infrared (NIR) wavelengths, which deeply penetrate tissues. Localization of a contrast-enhanced target in three dimensions necessitates: (1) rapid and precise acquisition of fluorescence measurements; (2) accurate prediction of these measurements using an appropriate theoretical model of light propagation through tissues; and (3) tomographic reconstruction of model parameters, which include optical and fluorescent properties of tissues, using an optimization routine that minimizes the differences between the experimental and predicted fluorescence data. Steady-state and time-resolved experimental measurements of diffuse fluorescence have been used to localize a contrast-enhanced target. The steady-state measurements provide information regarding the spatial origin of fluorescent photons, and the time-resolved measurements provide additional information regarding fluorescence decay kinetics, which may render functional information about the environment in which the fluorophores reside.
Experimental measurements of diffuse fluorescence acquired using point illumination and point detection techniques have been incorporated into tomographic reconstruction algorithms. In order to adequately probe a tissue volume using these techniques, illumination and detection must occur via fiber optic at several points about the tissue volume. Disadvantages with point illumination and point detection include (1) possible failure to sufficiently excite the contrast-enhanced target and (2) the production of sparse data sets, which make the underdetermined inverse problem difficult to solve.